Drive Circuit For A Power Switch Component

ABSTRACT

A drive circuit for a power switch component.

TECHNICAL FIELD

The present invention relates to a driving circuit for driving, forexample, a power mosfet.

BACKGROUND AND RELATED ART

For gate driving of large power train components, such as power mosfets,a number of solutions are known in the art. For example a number ofdifferent integrated circuits exists that are suitable for this purpose.These devices are expensive, and have specific properties, so that theyare not interchangeable. This means that when a particular type ofintegrated circuit from a particular vendor has been selected for anapplication it cannot easily be replaced by another type of integratedcircuit.

Drive circuits based on bipolar transistors are also known. Such drivecircuits are known to be more complex to achieve the same result.

There is a desire to achieve a negative voltage on the gate of thetransistor that drives the output signal to saturate it completely. Inthe prior art solutions based on bipolar transistors or integratedcircuits this is achieved by adding an additional voltage source forproviding the negative voltage, or using inverted driving stages incommon emitter style, which results in a more complex design.

SUMMARY OF THE INVENTION

The invention aims to provide a more flexible drive circuit for a powerswitch component.

According to the present invention a drive circuit for a power switchcomponent is provided, said drive circuit comprising an input terminalfor receiving a drive pulse, a first semiconductor component and asecond semiconductor component, each semiconductor component having acontrollable terminal, a voltage amplifying terminal and a currentamplifying terminal, the current amplifying terminals of said first andsecond transistors being interconnected, the voltage amplifying terminalof the first semiconductor component being connected to a first powerrail and the voltage amplifying terminal of the second semiconductorcomponent being connected to a second power rail, the first power railbeing at a higher potential than the second power rail, the controllableterminals of the first and second semiconductor components beinginterconnected, a resistor and a diode being connected in parallelbetween said controllable terminals and the input terminal, and acapacitor being connected between said controllable terminals and saidcurrent amplifying terminals, wherein each of said semiconductorcomponents has a diode connected between its current amplifying terminaland its voltage amplifying terminal.

The drive circuit according to the invention is based on transistors,which are substantially equivalent independently of the manufacturer orsupplier. Hence, the selection of a drive circuit does not result independency of a particular type of component from a particular vendor.Also, the drive circuit can be implemented at a relatively low cost.

With the drive circuit according to the invention a saturated drivevoltage to both negative and positive supply rail voltage can beachieved using only one power supply rail. The output voltage can bedriven up to the rail voltage and down to the negative rail voltage evenif the drive voltage is lower than the positive rail voltage, andnegative rail voltage, using only one power source. The delay time ofthe drive circuit is adjustable. Further, the change in current withrespect to time, di/dt, and the change in voltage with respect to time,dv/dt, can be made very fast.

In a first preferred embodiment the first and second semiconductorcomponents are mosfets. In this case, the controllable terminals are thegates of the mosfets, the voltage amplifying terminals are the drainsand the current amplifying terminals are the sources. The diodes arebody-drain diodes, which are inherent in the mosfets.

In a second preferred embodiment the first and second semiconductorcomponents are bipolar transistors. In this case the controllableterminals are the bases of the transistors, the voltage amplifyingterminals are the collectors and the current amplifying terminals arethe emitters. Bipolar transistors do not have inherent body-drain diodesin the way that mosfets do; therefore, separate diodes, connected in thesame way as the body-drain diodes of the mosfets, must be provided whenusing bipolar transistors.

In a third embodiment the first and second semiconductor components areIsolated Gate Bipolar Transistors (IGBT). In this case the controllableterminals are the gates of the transistors, the voltage amplifyingterminals are the drains and the current amplifying terminals are thesources. As for the bipolar transistors, separate diodes connected, inthe same way as the body-drain diodes of the mosfets, are provided.

The potentials of the first and second power rails may be selected indifferent ways. For example, the second power rail may be neutral whilethe first power rail has a positive potential. Alternatively, the firstpower rail may neutral and the second power rail has a negativepotential. The important thing is that the first power rail has a higherpotential than the second power rail.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, by wayof example and with reference to the appended drawings in which:

FIG. 1 illustrates a drive circuit according to a preferred embodimentof the invention, driving a mosfet.

FIG. 2 illustrates the voltage variations at different points of thedrive circuit before, during and after a voltage pulse is applied to theinput of the drive circuit.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates a drive circuit according to a preferred embodimentof the invention. The drive circuit is arranged in this example to drivea power switch component 3. The driven component could be, for example,a mosfet, a bipolar transistor or, an Isolated Gate Bipolar Transistor(IGBT). The drive circuit comprises a first, n-channel, mosfet 5 and asecond, p-channel, mosfet 7 that are connected to each other by thegates and the sources. Diodes 9, 11, referred to as body-drain diodes,from the source to the drain of the n channel mosfet 5 and from thedrain to the source of the p channel mosfet 7, are inherent in themosfets. The drain of the n-channel mosfet 5 is connected to a positivebranch 13 of a power source. The drain of the p-channel mosfet 7 isconnected to a neutral branch 15 of the power source. The gates of themosfets 5, 7 are connected to a first end of a resistor 17 connected inparallel with a diode 19 whose anode is facing the gates. The other endof the resistor 17, and the cathode of the diode 19 are connected to aninput terminal 21 arranged to receive a control signal for the drivecircuit 1. Between the first end of the resistor 17 and the anode end ofthe diode 19 on one side and the sources of the mosfets 5, 7 on theother side a capacitor 23 is connected.

It should be understood that instead of a positive rail 13 and neutralrail 15, the rails 13, 15 could have any potential as long as the firstrail 13 has a higher potential than the second rail 15. For example, therail 13 could be neutral and the rail 15 could have a negativepotential. Hence, the terms “positive” and “neutral” in relation to thepower rails 13, 15 in this description could be replaced by “higherpotential” and “lower potential”.

When a positive pulse, referred to the neutral branch 15 of the powersource, is provided on the input terminal 21 the gate voltage of then-channel mosfet 5 will increase, and it will start to conduct. By thetime the mosfet 5 starts to conduct the same time the capacitor 23 willalready be charged, and will bootstrap the voltage back to the gates andsaturate the n-channel mosfet. This will give the output side of thebuffer at the interconnected sources of the mosfets 5, 7 a very cleanand rapid drive up to the positive rail 13 of the power source. Thesaturated drive will continue as long as the capacitor 23 is notdischarged through the resistor 17. That means that the maximum drivevoltage to the gate of the power switch 3 is applied when needed toovercome the heavy charge to fully saturate in short time, thus minimizeswitching losses of the power switch 3. The short distinct delayprovided by the capacitor 23 and resistor 17 before the n-channel mosfet5 starts to conduct can be used to prevent cross-conduction in differenttypes of dc/dc converter topologies. Because of the bootstrap providedby the capacitor 23 a large positive feedback with a high dynamic gainwill be provided, thus minimizing the positive slope time, insteadgiving a fixed delay of the signal.

When a negative going signal is applied at the input terminal 21 thediode 19 will start to conduct and drain gate of the n-channel mosfet 5so it will stop conducting and when the voltage drops more the p-channelmosfet 7 will start to conduct and drain the gate of the power switch 3.At the same time the capacitor 23 will drain the gate of the p-channelmosfet 7 and conduct a current so that a negative going voltage overgate to source over p-channel mosfet 7 is developed. This will saturatethe negative going output signal of the driving stage completely tolower potential branch 15 at the output of the drive circuit. Thedynamic gain is high due to the large positive feedback capacitor 23 isapplying. This results in a fast going negative slope that goes tonegative voltage. The nature of the negative going slope is of great useto ensure that the driven component 3 is properly drained. This isuseful both for bipolar and mosfet transistors.

In the embodiment shown in FIG. 1 mosfet transistors 5, 7 are used inthe drive circuit 1. As mentioned above, such transistors have inherentdiodes between the source and the drain. Instead of mosfet transistors,bipolar transistors could be used.

In this case, diodes would have to be provided between the emitter andthe collector, in a corresponding way to the body-drain diodes 9, 11illustrated in FIG. 1. FIG. 2 show three curves a, b and c, which aretypical curves of the voltage between lower potential branch 15 andsignal processes in the circuit. The vertical lines labelled t1-t3signify points in time where different events occur in the circuit.

Curve a illustrates the signal over input terminal 21 referred to lowerpotential branch 15. At a first point in time t1 a positive going signalrelative to the lower potential branch 15 is applied to the inputterminal 21. At a third point in time t3 the signal on the inputterminal 21 goes negative again.

Curve b illustrates the voltage on the gates of the n-channel mosfet 5and the p-cannel mosfet 7 relative to the lower potential branch 15. Atthe first point in time t1, when a positive going signal referred tolower potential branch 15 is applied at input terminal 21, capacitor 23starts to charge through the resistor 17 making a positive going slope.The time it takes to reach the conducting threshold of n-channel mosfet5 is referred to as a dead time. When the n-channel mosfet's 5conducting threshold is reached it starts to conduct, at the secondpoint in time t2. Capacitor 23 will now act as a positive feedbackelement and bootstrap a positive voltage higher than the signal at inputterminal 21 back to n-channel mosfet 5 gate and make it fully saturateto positive branch 13. This will cut down the switching time, thereforea very sharp positive going signal between lower potential branch andgate of the n-channel mosfet 5 is observed for the gate of the powerswitch 3. A fully saturated signal will be available as long ascapacitor 23 does not discharge too much through the resistor 17. At thethird point in time t3 the signal on the input terminal 21 goes negativeand together with the body-drain diodes 11, 19 will create a negativegoing voltage over the gate of the n-channel mosfet 5 diode 19 speed upthe process.

When the p-channel mosfet 7 reaches its conducting threshold it startsto conduct and the capacitor 23 will now act as a positive feedbackelement and bootstrap a negative voltage relative to the negative signalat input terminal 21 back to the p-channel mosfet 7 gate and make itfully saturate to negative branch 15. This will cut down switching time,therefore a very sharp negative going signal between higher potentialbranch 13 and gate of p-channel mosfet 7 is observed for the gate of thepower switch 3.

Curve c illustrates the signal at the gate of the power switch 3referred to lower potential branch 15. As can be seen, at the secondpoint in time t2 the gate voltage of the power switch 3, that is, theoutput voltage from the driving circuit 1, increases sharply and staysat a high level until the third point in time t3, when the signal on theinput 21 becomes low.

As will be appreciated, the signals shown in FIG. 2 are ideal signals.In reality a certain amount of time is needed for a signal to becomehigh or low. Using the driving circuit according to the invention,however, these times are significantly reduced compared to the priorart.

1.-6. (canceled)
 7. A drive circuit for a power switch component saiddrive circuit comprising: an input terminal for receiving a drive pulse;a first semiconductor component; a second semiconductor component; eachof the first semiconductor component and second semiconductor componenthaving a controllable terminal, a voltage amplifying terminal and acurrent amplifying terminal, the current amplifying terminals of saidfirst and second semiconductor components being interconnected, thevoltage amplifying terminal of the first semiconductor component beingcoupled to a first power rail and the voltage amplifying terminal of thesecond semiconductor component being coupled to a second power rail, thefirst power rail being at a higher potential than the second power rail,the controllable terminals of the first semiconductor component andsecond semiconductor component being interconnected; a resistor; adiode, the resistor and diode being coupled in parallel between saidcontrollable terminals and the input terminal; a capacitor being coupledbetween said controllable terminals and said current amplifyingterminals, wherein each of the first semiconductor component and secondsemiconductor component has a diode coupled between its currentamplifying terminal and its voltage amplifying terminal.
 8. The drivecircuit according to claim 7, wherein the first and second semiconductorcomponents are metal oxide semiconductor field effect transistors(MOSFETs), the controllable terminals are the gates of the MOSFETs, thevoltage amplifying terminals are the drains and the current amplifyingterminals are the sources, and the diodes are body-drain diodes inherentin the MOSFETs.
 9. The drive circuit according to claim 7, wherein thefirst semiconductor component and the second semiconductor component arebipolar transistors, the controllable terminals are the bases of thetransistors, the voltage amplifying terminals are the collectors and thecurrent amplifying terminals are the emitters, and separate diodes areprovided between each current amplifying terminal and voltage amplifyingterminal.
 10. The drive circuit according to claim 7, wherein the firstsemiconductor component and the second semiconductor component areisolated gate bipolar transistors, the controllable terminals are thegates of the transistors, the voltage amplifying terminals are thecollector and the current amplifying terminals are the emitter, andseparate diodes are provided between each current amplifying terminaland voltage amplifying terminal.
 11. The drive circuit according toclaim 7, further comprising a first power rail and a second power rail,wherein the second power rail is neutral and the first power rail has apositive potential.
 12. The drive circuit according to claim 7, furthercomprising a first power rail and a second power rail, wherein the firstpower rail is neutral and the second power rail has a negativepotential.